Electron beam recording medium with acid sensitive indicator and halogenated polymer coating



United States Patent Ofifice 3,425,867 Patented Feb. 4, 1969 ELECTRONBEAM RliCORDING MEDIUM WITH ACID SENSITIVE INDICATOR AND HALOGEN- ATEDPOLYMER COATING Horatio Serafino Stillo, White Bear Lake, Minn.,assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn.,a corporation of Delaware No Drawing. Filed Sept. 23, 1963, Ser. No.310,922 The portion of the term of the patent subsequent to 7 Feb. 27,1985, has been disclaimed US. Cl. 117-230 Int. Cl. H011 15/00 Thepresent invention relates to media for reCOrding intelligence,particularly for the recording of intelligence by means of an electronbeam, and to the process in which such media are used.

Cathode ray tubes with phosphor screens having a visible responseessentially instantaneous with electron beam impingement have beencommonly used for transducing transmitted intelligence in electricalform into a visual form. However, the image on the phosphor screens ofsuch tubes is transitory and cannot be stored for any length of timewithout auxiliary means for providing a permanent record. Numerousefforts have been directed to recording permanently the transitoryimages on cathode ray tubes by various electronic and opticalphotographic methods. Special tube faces have been suggested to permitthe use of electrostatic development of special papers passedcontinuously over the tube face, but these tube constructions frequentlyhave produced records with poor resolution due to various forms ofdistortion associated with the system. The recording of a light image ona medium containing an acid-base type indicator dye and a halogenatedcompound which produces hydrochloric acid under the influence of light,has been described in the literature. However, applicant has found thatsuch media, when placed under vacuum conditions for direct electron beamimpingement, tend to lose volatile reactants, e.g. the relatively lowmolecular weight halogenated compounds.

It is an object of this invention to provide an electron beam recordingmedium which can be used under vacuum conditions and which can beactivated with direct electron beam impingement.

It is another object of this invention to provide an electron beamrecording media which can provide relatively stable storage ofintelligence over an extended period of time.

Still another object of this invention is to provide a recording mediumfor reliable and efiicient recording of images formed by an electronbeam.

Yet another object of this invention is to provide a process fortransducing electrical information into a visual record.

A further object of this invention is to provide means for recordingfacsimile or electrical intelligence.

Other objects and advantages will be apparent from the followingdescription.

In accordance with this invention the above and other objects arerealized by providing a recording media which comprises an electricallyconductive substrate on which is contiguously superimposed a layercontaining an acid sensitive indicator in basic form and a highlyhalogenated polymeric binder. Although the indicator is generallyhomogeneously disposed throughout the halogenated polymeric binder, andis preferably dissolved therein, it may also be provided as a localizedcoating on top or in the top surface of the polymeric binder.

Any acid sensitive indicator or pH sensitive system capable of effectinga color change at a pH of from about 1.5 to about 7 may be employed inits basic or unreacted form, the more preferable materials being thoseoil soluble materials which alter their color value in the pH range 6Claims of 2 to 4 and have relatively low volatility, e.g. a boilingpoint of above about 90 C. at atmospheric pressure. One preferred classof useful indicators includes those dyes which become quinoid in thepresence of acid, such as the amino azo dyes, eg dimethylaminoazobenzene (Methyl Yellow, CI. 19), characterized by their goodsensitivity and, in the case of Congo Red A, ability to form very stableimages. Color bases of diary] and triaryl methane dyes (e.g. Auramine)can also be used. Other illustrative dyes include the azines (e.g.Natural Red), phthaleins (e.g. Thymol Blue), hydroxyazo dyes (e.g.Salochrome Yellow 3G), etc. The oil soluble dyes are generally preferredbecause the presence of water solubilizing groups (e.g. acyl groups andtheir salts) tends to reduce the electron beam sensitivity of the film.Solubility of the indicator in the highly halogenated polymeric bindernormally permits optimum results with a minimum concentration ofindicator. Various other systems, including those in which two or morematerials react, e.g. by coupling, in an acid environment to form amaterial of another color, with or without oxidation, heat, etc. arewithin the scope of this invention. Where oxidation assists thereaction, exposure to the atmosphere frequently will accelerate thereaction, and in some instance the inclusion of an oxidizing agent inthe electron beam sensitive layer can be of value for this purpose.

The highly halogenated polymers which liberate hydrogen halide uponelectron beam bombardment should be normally solid and of sufficientlyhigh molecular weight to prevent their volatilization (i.e. above 1000preferably above 10,000 number average molecular Weight), be filmforming, and contain, in addition to hydrogen, from about 25 to about 73weight percent of labile chlorine or bromine. For ease of coating theelectrically conductive substrate the polymers desirably are soluble inconventional organic solvents such as tetrahydrofuran, acetone,2-butanone, methyl ethyl ketone, etc., although other solvent systemscan be used for the more difficultly soluble polymers such as polyvinylchloride and polyvinylidene chlorine. Solubility can be adjusted to someextent by employing copolymers, a balance being achieved between halogencontent and copolymer solubility. Vinylidene chloride copolymers withsuch monomers as the aliphatic acrylates (e.g. n-butyl acrylate, methylacrylate, ethyl acrylate, hexyl acrylate, methyl methacrylate,beta-chloroethyl acrylate, etc.), acrylonitrile, vinyl chloride, vinylacetate, vinyl butyrate, etc. are preferred highly halogenated polymersystems. Ethylenically unsaturated monomers with a high halogen content,such as l,1,3,3,3-pentachloropropene-l, fiuorotrichloroethylene,l,1-difiuoro-2,2-dichloroethylene, trichloroethylene, etc. copolymerizedwith vinyl or Vinylidene chloride or bromide or with the aliphaticacrylates can also be employed. Halogenated aromatic polymers areconsiderably less effective than the halogenated aliphatic polymers,although the copolymerization of a suitable halogenated aliphaticmonomer with an aromatic monomer (e.g. styrene, vinyl toluene, vinylcarbazole, etc.) selected for its solubility characteristics issuitable. With the preferred Vinylidene chloride polymers the chlorineconcentration ranges from about 25 to about 73 percent, preferably fromabout 40 to about percent by weight. With the vinyl chloride polymersthe chlorine concentration ranges from about 35 to about 55 percent,preferably from about 20 to about 55 percent by weight of the polymer.Although the halogenated polymers are desirably deposited from solutionas a film on the electrically conductive substrate, they may also bedeposited from a latex or intimate dispersion. With those polymers whichtend to decompose slowly in the presence of ordinary light andatmospheric oxygen, anti-oxidants and other stabilizers may be added toimprove good storage life.

Since the highly halogenated polymer serves as a relatively non-volatilesource of hydrohalic acid, no other brominated or chlorinated compoundswhich liberate acid under electron beam exposure are required in thefilm. In fact, the monomeric or non-polymeric halogenated compounds areundesirable not only because of their higher volatility in vacuum butalso because of their frequently encountered sensitivity to moisture,air oxidation and photodecomposition under visible light and theiradverse effect on the physical properties of the film, such as. bydilution of the polymeric vehicle. Materials which are highly sensitive,such as silver halide, Zinc oxide, etc., are also undesirable, since themedia construction should be essentially stable to visible light. Theslight fading of certain indicators upon extended light exposure, ishowever, unobjectionable. These problems are eliminated and thereliability of the recording sheets is improved by excludingnon-polymeric sources of hydrogen halide and visible light sensitivematerials.

The media of this invention may be prepared by mixing a minor amount ofthe acid sensitive indicator system with a solution of the highlyhalogenated polymer and coating the resulting admixture as a thin film,i.e. usually from 1 micron to several mils, preferably from about 2 toabout 15 microns, onto the electrically conductive substrate. Aluminumfoil, metal coated plastic, conductive papers, etc, can be used for theelectrically conductive substrate. If a transparency is desired or if itis desired to view the image through the backing, a thin, lighttransmissive aluminum vapor coated plastic (e.g. polyethyleneterphthalate) or a conductive glass (e.g. Nesa glass) substrate may beused in conjunction with a light transmissive, electron beam sensitivecoating. Many of the highly halogenated polymers may be made even morerelatively light transmissive in the form of a thin film. For eachequivalent weight of acid sensitive indicator from about 1 to about 1000acid equivalents of the halogenated polymer are employed, although theratio of these ingredients varies with the particular indicator system,and its acid sensitivity, which is employed. Other additives, e.g.plasticizers, oxidizing agents, etc. may be incorporated into theelectron beam sensitive coating, provided they do not serve to liberateacid under the electron beam. Additional films or coatings may beprovided on the electron beam sensitive layer to protect it fromabrasion, etc., provided they are relatively transmissive to theelectron beam.

With the media described above a color change is generally observed inthe sensitive coating immediately upon electron beam impingement orshortly thereafter upon exposure to air, thereby providing a visiblerecord. In some instance, when the acid sensitive indicator isreversible, as with the acid-base indicator dyes, the image can beerased by heating the media to about 100 C. to 150 C. for approximately30 seconds, the color change being probably due to the volatilization ofthe acid and an increase in effective pH of the media. Erased media ofthis type can be reused for electron beam recording, although subsequentdepletion of the polymeric acid source eventually reduces the efiiciencyof the recording.

It is also within the scope of this invention to impinge the electronbeam on a media comprising the highly halogenated polymer film on anelectrcally conductive substrate, leaving the indicator out of thepolymer film. After exposure to the electron beam, the liberated acid inthe imaged areas can be subsequently developed by contacting the exposedsurface with the acid sensitive indicator system. A separate developmentroller or bath may be used for this post development step, or a secondfilm construction incorporating or carrying the indicator can be broughtinto contact with the exposed surface of the media. One advantage of apost development using an indicator-containing film is the ability toprepare multiple copies.

If the electron sensitive layer can be supported on and contiguous withan electrically conductive roll or plate during beam exposure, theelectrically conductive substate can be eliminated as an integral partof the recording media construction. However, good electrical contactbetween the electron sensitive layer and a conductive backing, normallymaintained at ground potential, is best achieved with an integral orunitary media construction including an electrically conductive layercontiguous with the electron beam sensitive layer.

Although the foregoing description has been directed to electron beamrecording, the media described are also sensitive to other forms ofrelatively high energy irradiation which causes the liberation ofhydrohalic acid from the highly halogenated polymer, and alpha, beta andgamma radiation as well as X-rays and ion particles and ultravioletlight below 3,000 angstroms may also be employed at appropriate energylevels.

Since ultraviolet light below 3,000 angstroms is similar to an electronbeam in its effect on the recording media of this invention, a simplestandard test procedure has been developed to assist in the selectionand definition of the preferred highly halogenated polymers andindicator systems for use in the electron beam sensitive layers of themedia. The suitability of a film forming halogenated polymer isdetermined by adding 5 milligrams of Congo Red A to 1.0 milliliter of a20 weight percent solution of the polymer in a suitable solvent, such astetrahydrofuran. This solution is then knife coated onto a celluloseacetate, polyethylene terephthalate or glass backing to provide a dryfilm of 0.1 mil thickness. A sample of this dry film is placed at asuificient distance from an ultraviolet light source to provide about0.08 watt per square centimeter of radiant energy of 2000 to 3000angstroms wavelength. The sample is irradiated for a period from 2 to 30seconds. Generation of a blue color indicates a halogenated polymercontaining labile halogen useful in the electron beam recording media ofthis invention. The same standard test procedure is modified forselection of a suitable acid sensitive indicator by using a 20 weightpercent solution of vinylidene chloride-acrylonitrile copolymer 10 molratio) and 5 milligrams of the acid sensitive indicator system, a strongcolor change after the ultraviolet exposure indicating a usefulindicator for the electron beam recording media.

The following examples will serve to illustrate the recording media andprocesses of this invention.

Example 1 To 4 ml. of a 20% (by weight) solution of vinylidenechloride/n-butyl acrylate copolymer (90/ 10 mol ratio, approximately 60%chlorine) in tetrahydrofuran, was added 10 mg. of Congo Red A dye(unsulfonated Congo Red prepared from tetrazotized benzidine andl-naphthyl amine) and 4 drops of cyclohexanone. This solution was usedto prepare a film of 0.1 mil dry thickness when it was knife coated onto3 mil aluminum vapor coated polyethylene terephthalate film.

An electron beam recording made with a scanning rate of 60 fields/sec.,10 kv., 5 microampere beam current, 10 micron spot size and 50microsecond/scan line (2625 lines/field) produced a direct print outblue image on an orange background. The individual scan lines weredistinctly visible under a microscope. The change in optical densitybet-ween background and image was about 0.5 optical density units.

Congo Red A has the following structure:

NHZ NH:

Example 2 To 4 ml. of a 20% (by weight) tetrahydrofuran solution of apolymer of vinylidene chloride/n-butyl acrylate (92/ 8 mol ratio,approximately 62% chlorine) was added 20 mg. of Congo Red A dye and 4drops of cyclohexanone. This solution was used to prepare a film of 0.1mil dry thickness by knife coating into 3 mil aluminum vapor coatedpolyethylene terephthalate rfilm. When it was recorded as above asimilar image was obtained having an optical density difference of 0.6between background and image.

Example 3 Films prepared and recorded as above using 20% tetrahydrofuransolutions of vinyl chloride/vinyl acetate copolymer (43% by weightchlorine); vinylidene chloride/ acrylonitrile copolymer (54% chlorine);vinylidene chloride/acrylonitrile copolymer (67% chlorine); tosylatedpolyvinyl alcohol; chlorinated polypropylene; copolymers of vinylidenechloride with any of methyl acrylate, ,B-chloroethyl acrylate, vinylchloride, vinyl acetate or vinyl butyrate, into each of which 1-20= mg.of Congo Red A dye were added (per 200 mg. polymer) yielded direct printout images. The optical density difference between image and backgroundvaried depending on the specific polymer and the amount of dye used.However, the films all gave differences lying in the range of 0.1-0.3optical density unit.

Example 4 To 4 ml. of a 20% (by weight) solution of a polymer ofvinylidene chloride/n-butyl acrylate (90:10 mol ratio; approximately 60%chlorine) in tetrahydrofuran was added 20 mg. of 4-phenylazo-diphenylamine and 4 drops of cyclohexane. This solution was used to prepare afilm of 0.1 mil dry thickness when it was knife coated onto 3 milaluminum vapor coated Mylar film.

An electron beam recording made with a scanning rate of 60 fields/sec,kv., 5 microampere beam current, 10 micron beam spot and 50 microsecond/scan line showed a direct print out, brown image on a pale yellowbackground. The individual scan lines were distinctly visible under amicroscope. The change in optical density between image and backgroundwas about 0.5 optical density units.

Example 5 Using a vinylidene chloride/n-butyl acrylate copolymer (about60% chlorine) and the same procedure for coating and recording asoutlined above in Example 1 the following amino azo dyes were tested:

A recording medium having outstanding characteristics was prepared byadding 0.030 g. of Congo Red A to 3.5 ml. of a solution of 5.00 g. of acopolymer of 87% vinyl chloride and 13% vinyl acetate dissolved anddiluted to 100 ml. with methylethyl ketone. To this solution was added1.5 ml. of methylethyl ketone, 4.0 ml. of n-amyl acetate and 1.0 ml. of1,1,2,2-tetrachloroethane (these last two solvents are added to decreasethe evaporation rate and to obtain unblushed coatings). The resultingclear red solution was then dip coated onto aluminum foil at a rate of43 ft. per second. The coated foil was pulled vertically into a 1% inchdiameter tube where it was allowed to dry. Solution 1.25 to 5.00 g. per100 ml. solvent gave dry coating thickness of 3 to 5 microns. The dried,coated foil was then evaluated for sensitivity using an electron gunhaving an unmodulated, non-sweep beam from about 6 2,000 to 15,000 voltsand beam currents up to about 50 microamperes. The spot size was about0.50 mmF. A

sensitivity of about 0.1 microampere-second/mm; was

obtained. This corresponds to a recording sensitivity of about 4megacycles per second.

Media using other concentrations of polymer and Congo Red A wereprepared and successfully tested for for electron beam sensitivity.

The n-amyl acetate and tetrachloroethane added to eliminate blushing ofcoatings may be replaced by other materials such as cyclohexane ordimethyl Cellosolve.

Examples 7-24 Aluminum foil was dip coated in the manner described inExample 6 to provide coatings having about 1 part of indicator to 5parts of polymer at from 3 to 5 microns dry film thickness. The moresensitive films had a lower value of marking energy expressed asmicroampere-secJmmF. The polymer was vinyl chloride/vinyl acetate (87/13mol ratio) except where indicated.

Marking Example Coating energy (microampsee/mm!) 3 100 100 70 60 20 1520 16 Bromocresol Purple made alkaline 10 by NH: exposure. 17Bromophenol Blue made alkaline by 50 NH: exposure. 18 Congo Red 10Brilliant Paper Yellow. 20 Neutral Red (azlne) 80 Indophenol Blue 30 22Salochrome Yellow 36 (hdroxyazo) 50 23 Induline Base B 20 24. Nile BlueRYA 10 In general, those acid sensitive indicators with a marking energyof below 10 microampere-seconds/mmfi, preferably below 0.3microampere-second/mmfi, are superior for the recording sheets of thisinvention.

Example 25 1.0 by weight Victoria Green WB Base and 10% of a copolymerof vinyl acetate/vinyl chloride and 89 weight percent of acetone wascoated onto '1 mil aluminum foil at a wet coating thickness of 1 mil.The coating was dried to a slightly yellow, clear film (leuco reaction).A 20 frame exposure to an electron beam -(19 kv. and -125 microamperes,0.3 micron vacuum) produced a green positive image on a clearback-ground.

Example 26 1.0% Victoria Green WB Base and 10% of a copolymer ofvinylidene chloride and acrylonitrile and 89 percent by weight ofacetone was coated onto 1 mil aluminum foil at a wet coating thicknessof 1 mil. The coating was dried to a dark green color. A 10 frameexposure to an electron beam (19 kv. and 115-125 microamperes, 0.3micron vacuum) produced a reddish purple positive image on a greenbackground. Rhodamine B Base can be used instead of Victoria Green WBBase.

The following examples illustrate further indicators and their use inthe electron beam recording media of this invention. The highlyhalogenated polymeric binder was a copolymer of 87 mol percent vinylchloride and 13 mol percent vinyl acetate, the weight ratio of indicatorto binder being 1 to 10. Indicator and binder were either dissolved inmethylene chloride (for the more soluble indicators) or dispersed byball milling for four hours or less. Coatings were prepared by knifecoating onto a metal substrate to provide a dry film thickness of 0.1mil. Table I sets forth these results. The electron beam exposure isexpressed as fields (60 fields per second).

TABLE I Electron beam exposure at Image Image before Color after Ex.Indicator pH for color change at TV rate of 30 frames/sec. qualityexposure exposure 27'.... Crystal Violet l.8 30 fields (30 kv., 100amp). Fair Blue Yellow. 28.-. Methyl Violet 1. 6 30 fields (30 kv.,16014 amp) .do. do. Do. 29"... Malachite Green. do .do Green. Do. 30-..Homorosanilinm- 30 fields (25 kv., 100;; amp). Poor Red- Faint yellow.31 Methyl Yellow 1 field kv., 35,1 amp).... Good Yellow Red. 32 CongoRed (dispersed) 3.0-5.0 1 field (20 kv., 100; amp) l do. Red... Blue.33.-. 4,4-rnethylenebis N-dimethylaniline About 6 (with oxidation) 1field (20 kv., 351.: amp) Excellent.. Colorless Do. 34... Methyl Red(dispersed) 4.8-6.0 1 field kv., 60;: amp)..." Good".-- Faint yellowRed. 35. Phenolsulfone (phthalein)... Gil-8.0.-.. 1 field kv., 160;;amp) "do..." Red Yellow. 36 Crystal Violet (leuco base), About 6.. 1field (15 107., 40; amp).. Excellent. Colorless Blue. 37"-..Pararosaniline (carbinol base) About 6 1 field (20 kv., 1 amp) do Slightpiuk. Red. 38 Diphenylamine 1.0 (with oxidation). 30 fields (30 kv.,160;: amp)" Fair Colorless Yellow. 39... Methylene B1ue.. 1.0 d d BColorless. 40w. Sevron Yellow R Green.

Sevron Red Red. 42... Tetramethylbenzldene Yellow. 43--." Sairanine T o.44 -Diphenylarnine sulfonic acid barium teen Colorless. 45. indsohedlersGreen -e.do .do..-.. Light green..- Dark green.

In the examples of Table II the preferred indicators, i.e., amino azocompounds are employed. The highly halo- 20 chloride and 12 mol percentvinyl acetate, the weight ratio genated binder was a copolymer of 88 molpercent vinyl of indicator to binder being about 1 to 5.

TABLE II.-AMINO AZO DYES IN ELECTRON SENSITIVE MEDIA Ex. IndicatorEnergy to mark (micro- Color response amperes-sec. at 2,000 volts) C H:A. ON=NC N\ 0. 2 Red on yellow.

B. --N=NNH1 03 Do.

I C H:

C 0 O H (Methyl Red) NH: NH: D N=N-C -C 10 Blue on red, poor solubility.m (U SOzH S 0 H NH: H!

E 2 Blue on red, ex-

cellent stability.

N=N NH; 1 Red on yellow.

11..... w 1 Blue on red.

Ex. Indicator Energy to mark (micro- Color response amperes-sec. at2,000 volts) N CH;C N= -NHl 03 Purple on yellow.

o no OON=N-N(CHI)1 1 Red to yellow.

P CH;O N=NO-NH| 03 Purple on orange.

Q oH@0- NH: .2 Blue on purple.

SO Na R HQ O QQ =N NH| 03 Blue on red.

C Ha O C H:

OH CM MMGNH, .03 on T CHaOC N=N- N(C:Hs): 40 Orange on yellow.

ON=NO-N .02 Purple on yellow.

CHaO-C N= ONH- NO: .33 Do.

IfHz

W ClH5O- N=N NH; 2 Brown on tan.

a) C Ha X CH N= NH; .2 Pale yellow on tan.

C Ha

ornoO-N: NH: .2 Purple on yellow,

excellent stability.

orno-O-rwrx-Qou .2 Red on tan.

H O I \l=N O C H:

. 2 Yellow on tan.

Various other embodiments of the present invention will be apparent tothose skilled in the art without departing from the scope thereof.

The following is claimed:

1. A recording medium which is stable under vacuum conditions used inelectron beam recording and which comprises an electrically conductivemetallic substrate and, superimposed thereon, an acid-sensitiveindicator capable of changing color at a pH below about 7 and, as thesole remaining constituent capable of liberating hydrogen halide underexposure to an electron beam, a normally solid, highly halogenatedpolymer having a molecular weight of at least about 1000 and having atleast 25 weight percent of labile halogen selected from the groupconsisting of chlorine :and bromine.

2. The recording medium of claim 1 in which said film has from about 1to about 1000 acid equivalents of said (References on following page) 1112 References Cited 3,238,020 3/1966 Eiseman 23-253 UNITED STATESPATENTS 3,252,835 5/1966 NiSSCl et a1 117-34 X 3 x 5; WILLIAM D. MARTIN,Primary Examiner. 11/1949 f 51,, 5 EDWARD J. CABIC, Assistant Examiner.12/1955 Law .11734 X US. Cl. X.R 5/1956 Saulmer 25065 10/1958 James X117161, 201, 211, 217, 346-1, 135

1. A RECORDING MEDIUM WHICH IS STABLE UNDER VACUUM CONDITIONS USED INELECTRON BEAM RECORDING AND WHICH COMPRISES AN ELECTRICALLY CONDUCTIVEMETALLIC SUBSTRATE AND, SUPERIMPOSED THEREON, AN ACID-SENSITIVEINDICATOR CAPABLE OF CHANGING COLOR AT A PH BELOW ABOUT 7 AND, AS THESOLE REMAINING CONSTITUENT CAPABLE OF LIBERATING HYDROGEN HALIDE UNDEREXPOSURE TO AN ELECTRON BEAM, A NORMALLY SOLID, HIGHLY HALOGENATEDPOLYMER HAVING A MOLECULAR WEIGHT OF AT LEAST ABOUT 1000 AND HAVING ATLEAST 25 WEIGHT PERCENT OF LABILE HALOGEN SELECTED FROM THE GROUPCONSISTING OF CHLORINE AND BROMINE.